1. Field of the Invention
The present invention relates generally to the field of communication systems, and particularly to a method and apparatus for transmitting/receiving data with a time switched transmission diversity (TSTD) function.
2. Description of the Related Art
In a mobile communication system, data transmission/reception performance can generally be enhanced by utilizing diversity techniques in a fading environment. Typically, as shown in FIG. 1 three diversity techniques are applicable to the forward link and a single diversity technique (i.e., receiver diversity) is applicable to the reverse link. Data can be received on the reverse link with receiver diversity by equipping a base station with a plurality of reception antennas. For the forward link, the three well known diversity techniques include transmission diversity, receiver diversity, and mixed diversity. In transmission diversity, a base station transmits a signal through a plurality of transmission antennas and a mobile station receives the signal through a single reception antenna to achieve the same effect as if multiple reception antennas were used. Receiver diversity is provided when the mobile station has a plurality of reception antennas, and mixed diversity is defined as a combination of the two aforementioned techniques.
Receiver diversity on the forward link, however, is problematic in that diversity gain is low because of the small terminal size which limits the distance between reception antennas. Another problem is that the use of multiple reception antennas requires a separately procured hardware configuration for receiving a forward link signal and transmitting a reverse link signal through a corresponding antenna, thereby imposing constraints on the size and cost of the terminal. In view of these problems, mobile communication systems typically employ transmission diversity exclusively on the forward link.
FIG. 2 illustrates a general block diagram of a mobile communication system employing transmission diversity on a forward link. A base station 100 and a mobile station 200 include transmitting and receiving apparatus, respectively. A baseband signal processor 103 of base station 100 converts user data for transmission on the forward link into a baseband signal. Such conversion by baseband signal processor 103 includes channel encoding, interleaving, orthogonal modulation, and PN (Pseudo Noise) spreading. A signal distributor 102 distributes the signals received from the baseband signal processor 103 into N signal streams with each stream being provided to one of N transmission antennas TXAI to TXAN. As a result, transmission diversity is achieved at the transmission end of the base station 100 through the N antennas.
The mobile station 200 has a single reception antenna RXA for receiving signals from the base station 100 from the N transmission antennas. To process the received signals, the mobile station 200 includes N demodulators 201 to 20N corresponding to each N transmission antenna. A combiner 211 combines demodulated signals received from the demodulators 201 to 20N, and a decoder & controller 213 decodes a signal received from the combiner 211 to produce decoded user data.
In contrast, the structure of a transmitter in a non-transmission diversity (NTD) CDMA communication system is described with reference to FIG. 3. A base station 300 includes a CRC (Cyclic Redundancy Check) generator 311 for adding CRC bits to input user data in order to detect a frame error which occurs while sending the user data. A tail bit generator 313 adds tail bits indicating termination of a data frame to the data frame prior to channel encoding. Then, a channel encoder 315 encodes the data frame for error correction and an interleaver 317 interleaves the encoded data. A combiner 323 performs an exclusive-OR operation on the interleaved data with a long code sequence. This long code sequence is generated in a long code generator 319 and decimated in a decimator 321 at the same rate as that at the output terminal of the interleaver 317. A signal mapper 325 converts 0s and 1s of the encoded data received from the combiner 323 to +1s and −1s respectively, for orthogonal modulation. A serial-to-parallel (S/P) converter 327 divides the signal received from the signal mapper 325 into I channel and Q channel streams, for QPSK (Quadrature Phase Shift Keying) modulation. The I channel and Q channel streams are subject to orthogonal modulation in multipliers 329 and 331 and PN spreading in a PN spreader 333. The spread signals are filtered for pulse shaping in LPFs (Low Pass Filters) 335 and 337, loaded on a carrier by mixers 339, 341, combined with combiner 343, and finally transmitted through a transmission antenna.
The transmit signal which is output from the NTD transmitter in the base station 300 illustrated in FIG. 3 has a signal structure indicated by 511 of FIG. 5. Specifically, FIG. 5 illustrates timing characteristics for the case of transmitter diversity and no diversity. Specifically in the case of no diversity, FIG. 5 illustrates user data output from the NTD 511 transmitter, and for the diversity case. FIG. 5 further illustrates timing characterization from an orthogonal transmission diversity (OTD) transmitter with two antennas, A & B (N=2).
FIG. 4 is a block diagram of an OTD transmitter with two transmission antennas (N=2). Improved performance of a forward link is achieved in the OTD transmitter by dividing information for one user into two or more streams and transmitting the divided data through the plurality of transmission antennas, as indicated by 513 and 515 of FIG. 5. The following description is conducted with the understanding that [Wm−Wm] is identical to [Wm{overscore (Wm)}].
The OTD transmitter, illustrated in FIG. 4, operates in the same manner as the NTD transmitter of FIG. 3, except for a serial-to-parallel conversion process. In the OTD structure, mapped data branches into N streams, corresponding to the number of transmission antennas in S/P converters 413, 415, and 417, and orthogonally modulated in multipliers 419, 421, 423, and 425, for maintaining mutual orthogonality between the transmission antennas.
In addition to orthogonal modulation, orthogonal codes may be further utilized to ensure mutual orthogonality among the N antennas. The orthogonal code extension is accomplished by a Hadamard matrix extension. In the case of the OTD transmitter with two transmission antennas A and B(i.e., A and B as shown in FIG. 4) the different orthogonal codes assigned to the antennas are respectively [WmWm] and [Wm−Wm], extended from an orthogonal code Wm of a length 2m used in the NTD transmitter. The purpose of orthogonal code extension is to compensate for the data rate of each of the N streams, which is 1/N of the data rate prior to serial-to-parallel conversion.
A receiver for receiving a signal from the OTD transmitter requires signal demodulators for demodulating user data, a pilot demodulator for providing timing and phase information to be provided to the signal demodulates, and a parallel-to-serial (P/S) converter for converting M signal demodulator outputs to a serial signal stream.
A pilot channel is used by the base station to provide timing and phase information to a mobile station. The mobile station first activates the pilot demodulator to acquire necessary timing and phase information and demodulates user data based on the acquired information. For an OTD transmitter, each transmission antenna should be assigned a unique pilot channel.
In a receiver for use with a conventional OTD transmitter of FIG. 4, the pilot demodulator subjects a received signal to PN despreading and orthogonal demodulation and integrates the resulting signal for one cycle in order to demodulate a pilot channel from the received signal. A time estimator and a phase estimator in the pilot demodulator estimate timing and phase values from the integrated value.
A signal demodulator of the receiver performs PN despreading on a user data signal based on timing information received from the pilot demodulator. A phase error which occurs during transmission is compensated for by multiplying the phase information by an integrated value. The integrated value is obtained by integrating an orthogonally modulated signal for one cycle. The phase-compensated integrator output is converted to a probability value by a soft decision block and fed through the P/S converter to a deinterleaver.
Despite improved reception performance as compared to the NTD system, the conventional OTD mobile communication system has certain limitations. First, given that a terminal should be equipped with a number of pilot demodulators and signal demodulators corresponding to the number of transmission antennas of a base station, this results in an increase in the complexity, cost, and power consumption of a receiver.
Another drawback associated with a conventional OTD system is that the length of an orthogonal code used is increased by N times from that of an NTD case, for N transmission antennas. As a result, the integration interval is extended, thereby degrading reception performance in a frequency error-susceptible channel environment.
A further limitation is that the number of available transmission antennas is restricted to be a power of 2, namely 2n which imposes constraints concerning a number of applications involving antenna arrays. There exists a need, therefore, for a diversity scheme which overcomes the limitations of the prior art.